Doesn't load past a certain point

I created a superflat world and I made a few structures in it, and I wanted to make a copy of the world so I backed it up and all that. Anyway, when I logged back into the original, the world just stopped loading where I had structures built and most of what I built was inaccessible. This appeared in the copies, too so I'm not sure if my files got corrupted or if there's a glitch or what.